The present invention relates to a method of measuring the .sup.17 O isotope content of a body, and particularly to such a method using nuclear magnetic resonance (NMR) spectroscopy examination techniques. Possible applications of the novel method of the invention include measuring the rate of perfusion of a solution administered to a living body, and measuring oxidative metabolism activity in a living body.
Oxygen appears in nature in the form of three stable (i.e, non-radioactive) isotopes, one with predominant abundance .sup.16 O (99.76%), and two rare isotopes .sup.17 O (0.037%) and .sup.18 O isotope (0.20%). The isotopes .sup.16 O and .sup.18 O do not possess magnetic moment and therefore they do not have an effect on Nuclear Magnetic Resonance (NMR) spectroscopy, but the isotope .sup.17 O does have a magnetic moment and therefore can be detected by NMR. The .sup.17 O isotope also has an effect on other magnetic nuclei associated with it.
There are several applications where the detection of minute amounts of .sup.17 O by NMR is important. One is the measurement of the rate of perfusion by Magnetic Resonance Imaging (MRI): J. Pekar et al., Magn. Reson. Med. 21, 313 (1991); K. K. Kwong et. al., Magn. Reson. Med. 22, 154 (1991). This is accomplished by injecting physiological solution containing H.sub.2.sup.17 O and following the rate of distribution of the .sup.17 O isotope in the body. Another application is metabolic functional imaging: see J. Pekar et al., supra; also G. D. Mateescu et al., in "Synthesis and Applications of Isotopically Labelled Compounds"; T. A. Baillie and J. R. Jones Eds. Elsevier, Amsterdam, P.499 (1989). In this technique, molecular oxygen (O.sub.2) labeled with the .sup.17 O isotope is inhaled by the patient. Since O.sub.2 is a paramagnetic molecule, it cannot be observed by NMR. However, in metabolic active respiring tissues, it is converted to H.sub.2 O by oxygen metabolism.
There have been several suggestions in the literature (e.g., see above publications) of observing the .sup.17 O isotope directly by NMR. However, because of the low sensitivity of .sup.17 O NMR, and the high cost of .sup.17 O enriched compounds, this approach seems impractical for medical applications.
The present invention is directed to a novel magnetic resonance imaging (MRI) method based on indirect detection of the .sup.17 O isotope using .sup.1 H (proton) NMR. Conventional magnetic resonance imaging (MRI) is exclusively based on the detection of the echo signal produced by protons after being stimulated by a sequence of strong radio frequency (RF) pulses generated at the resonance frequency of the protons in a magnetic field. Since most of the hydrogen in living tissues is present in water molecules, the image produced by MRI reflects the distribution of water in the observed object. Because of the high natural abundance (99.985%) of .sup.1 H and its high gyromagnetic ratio, leading to high NMR sensitivity, proton MRI is by far the most commonly used MRI method.